In many analytical systems, discovering the nature of an unknown substance normally requires the substance to first be collected. There are detector systems that analyze a fluid flow analyte stream, i.e., vapors or gases, particulates and liquid bound analytes. Some detector systems are based, for example, on an optical analysis that determines analyte characteristics by subjecting a quantity of the analyte to a light beam and measuring the scattering or fluorescence effects. Chromatography detector systems, for example, are sometimes based upon the optical effects produced by analyte samples. There are both quantitative and qualitative analysis detector systems.
Before a sample may be analyzed by chromatography or by many other types of analytical techniques, the sample must be collected and then delivered to a chromatographic column or a detector system. Many samples of interest are available outside of a controlled setting. One important use for analyte analysis is for safety testing of environments that humans occupy. There is a heightened awareness in modern times of the potential for the intentional detonation of explosives or release of chemical or biological agents into environments occupied by humans. The environments might include open or enclosed spaces in work environments, public environments, or military environments, etc. Many building environments with ducted HVAC (heating ventilation and air conditioning) have the potential for the intentional release of TICS or chemical and biological agents into closed or open spaces occupied by military or civilian personnel. Manufacturing operations also have the potential to permit the escape of hazardous chemicals or biological agents into a manufacturing environment or to an external environment surrounding a manufacturing plant.
In some situations, detection may be desirable in a matter of seconds, but in others, an extended period of time may be used for collection before performing an analysis. An example of the latter case involves workers that may be exposed over a time period to unacceptable levels of harmful agents. Another example of the latter case is when cargo containers are transported from country to country by sea, it may be desirable to collect a sample over a period of several days prior to analysis.
In both uncontrolled settings and controlled settings, analytical resolution and the sensitivity of detection are dependent upon the efficiency of analyte collection and the efficacy of delivery of collected analyte to a detection system. It is desirable, for example, to detect very low levels of toxic or hazardous materials in a particular environment. Gas chromatography and other analytical techniques can employ a variety of detector types, and they have been demonstrated to be very sensitive types of analysis techniques, for example. Another example is a chemresistor based device, which uses a detector whose resistivity changes when it is exposed to particular chemical vapors. Whatever the type of detector system, however, concentrating analyte in a stage prior to the detector system can improve detection limits for the analyte(s) of interest, and can also provide a more reliable quantitative or qualitative determination of an analyte.
Constructing a portable field instrument for collection, storage, concentration, and possibly on-site analyte analysis also presents challenges. Compactness is an important factor to provide an instrument that is useful in the field, but one that competes with other design constraints in the case of a portable field instrument. Among other important factors are the sensitivity discussed above, the time scale required to collect and analyze a sample (preferably short), the amount of fluid flow that may be achieved (limited by tolerable pressure drops and pump capacity) while maintaining good analyte-sorbent material interaction, and the amenability of a device's collection hardware to be integrated with other parts of a field instrument. Low weight, durability, and low electrical power consumption are also desirable qualities for prolonged field use.
A known analyte collection and detector system arrangement uses a direct pneumatic sampling. A direct pneumatic sampling makes only a small portion of available analyte available to the detector system in any given time period. One technique of compensating for the low levels of analyte provided to the detector system involves taking a detector system signal over an extended time period. A typical strategy is to deliver a continuous or broad time pulse of analyte vapor to the detector system, e.g., a pulse extended from a few seconds to tens of seconds. This is employed with a detector system having relatively fast signal kinetics. An analyte signal is produced over an extended period of time, e.g., a broad flat curve lacking a sharp signal that may have a low and somewhat indistinct maximum value that is vulnerable to baseline shifts. The maximum signal value is dependent on the analyte concentration, the detector system characteristics and the time width of the vapor pulse sampled.
Others have worked on concentrating analytes, and have proposed systems including a micro scale collection section. A group working at Sandia National Laboratory in Albuquerque, N. Mex. has developed chemical preconcentrators including a preconcentrator heated plate that incorporates a sorbent material coating. This work is discussed, for example, in Manginell et al. U.S. Pat. No. 6,257,835, entitled Chemical Preconcentrator with Integral Thermal Flow Sensor and in Manginell et al. U.S. Pat. No. 6,171,378, entitled Chemical Preconcentrator. The chemical preconcentrator used in that work is formed from a substrate having a suspended membrane, such as low-stress silicon nitride. A resistive heating element is deposited over the membrane and coated with a sorbent, such as a hydrophobic sol-gel coating or a polymer coating. A fluid flow is passed over the sorbent to achieve a collection. A high concentration may then be delivered to a detector system by desorbing, which is achieved by heating the resistive heating element.
One advantage of this work by Manginell and others is that it can provide a relatively high concentration of analyte by collecting it over a long period, and then delivering it in a short amount of time. Another advantage is the MEMS (microelectromechanical systems) micro scale of the device and the MEMS fabrication techniques that permit integration of the device with other system components, for example to form a micro analytical system.
In another style of analyte collector, a column that is packed with a porous adsorbent is used to collect analyte by flowing air through the column and thermally desorbing collected material. The pressure drop associated with this sort of device is typically too high for high flow applications and requires higher power consumption. If the amount of adsorbent is minimized to allow higher flows or faster desorption, the dynamic range is compromised.
However, the inventors have recognized drawbacks in the known prior devices. With embodiments of the present invention, some or all of these drawbacks are overcome. One limitation of known prior devices is the reliance upon a flow arrangement where fluid flow during collection is generally passed over the sorbent in a direction generally parallel to its surface. Taking the example of an analyte vapor being passed over the sorbent coated preconcentrator of the Sandia work, much of the analyte vapor avoids contacting the sorbent in its flow over design. Increasing contact between the analyte fluid flow and the sorbent in the collection area would require creating a turbulent flow, a difficult task given the practical requirements including, for example, constraints such as a limitation on the amount of pressure drop. Flow could be increased, as well, but designs have generally looked away from high flow rates because the designs gravitate toward the generally low flow rates that provide optimal operating conditions for detector systems, such as gas column chromatography detector systems. In addition, if the dimensions of the pneumatic pathway that encloses the preconcentrator in the Sandia work were widened to afford a lower pressure drop across the device, and afford a higher flow rate capability, then the percentage of analyte vapor interacting with the sorbent would dramatically decrease. Artisans have typically viewed the flow rate of the detector system as a necessary limitation on a collection system, whereas embodiments of the invention permit collection flow rates well in excess of the flow rate that is well-tolerated by a detector system intended to be used in conjunction with preferred embodiment collection devices.